Increasing fuel efficiency of internal combustion engines and limiting their emission of undesirable combustion products are studied objects of engine control. Fuel efficiency is increased by limiting the amount of fuel provided to the engine during engine operation at constant or slowly changing engine speed under low engine loads. Providing a limited amount of fuel to the engine creates a mixture of air and fuel that contains more air than is required for combustion of the fuel. Such mixtures are referred to as a “lean mixture.” Combustion of lean air and fuel mixtures by an internal combustion engine, however, creates high combustion temperatures that result in formation of undesirable nitrogen and oxygen compounds, i.e., NOx pollutants.
When greater power is required from an internal combustion engine, either at constant engine speed, or more commonly, during operation to accelerate a vehicle, more fuel must be consumed to provide the required power. Meeting a demand for a quick increase in power output may require different fueling of the engine than during prolonged low power operation. For example, an enriched air/fuel that is at or near a stoichiometric air/fuel ratio may be delivered to the engine to increase the engine's power output. While the amount of NOx emissions may actually decrease with enriched air/fuel ratios, particulate emissions, e.g., soot, tend to increase during enriched air/fuel ratios.
Different fuel injection strategies, including the pattern and timing of fuel injections to an engine's combustion chambers, can be provided depending on the particular operating conditions an engine is experiencing. For example, one fuel injection strategy may be used when the engine is operating in a “steady state” condition, while a different strategy may be adopted during transient operations where engine power is increasing. By tailoring the fuel injection strategy to the engine operating condition, it is possible to address factors including fuel economy and pollution control, while still providing acceptable engine performance.